[Toybox] [PATCH] Fix ionice's return value for getting process IO priority

[enh]

[note: this isn't my patch; it was
https://android-review.googlesource.com/c/platform/external/toybox/+/3106282,
and i'm just forwarding it. the attachment is the original patch with
the original author's details, and i've cc:ed them. lgtm to me,
though, and matches the next function in the same file.]

In the user version, if you use ionice to get the process IO
priority without permission, -1 will be returned, but Idle: prio 7
will be printed at this time. This is an incorrect priority and
should return permission denied.
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